This invention relates to liquid phase sintered dense composite bodies typified by hard metals and cermets obtained by cementing hard refractory material, such as carbides, nitrides, oxides, borides, silicides, etc., with a metal and to methods for producing the same. More particularly, it relates to dense liquid phase sintered composite bodies containing a cementing metal component in very large amounts in limited sections or throughout the entirety of the composite and to a method for producing the same.
In producing a liquid phase sintered composite body, a cementing metal in the form of a fine powder having a grain size of from less than 1 .mu.m to several .mu.m is used, in order to facilitate the densification of the composite in the sintering process and to impart optimum properties to the sintered composite. Hard refractory material and cementing metal in powder form are uniformly mixed and compacted to produce a compact body which is subsequently sintered by heating to a sintering temperature and holding the compact body at the sintering temperature. In the liquid phase sintering process, the cementing metal component is melted and the compact body rapidly contracts due to the surface tension of the molten cementing metal component, thereby densifying the entire sintered body. The transformation of the cementing metal component into a liquid phase will be described in more detail. In the process of heating the compact body to a sintering temperature and holding it at such temperature, the elements of the hard refractory material in contact with the cementing metal diffuse in solid state into the cementing metal. This change in the composition of the cementing metal caused by the solid-state diffusion of the elements results in lowering of the melting point of the cementing metal component. It is well known that if the cementing metal forms a eutectic alloy with the elements diffused thereinto, then the cementing metal component is melted and densification of the compact body occurs upon heating the compact body to a temperature higher than the eutectic point. For example, in a liquid phase sintered composite of the WC--Co system, the melting point of cobalt metal is 1495.degree. C. However, since the cementing metal component of this composite has a eutectic point of about 1280.degree. C., sintering is liable to take place in a temperature range of 1350.degree. to 1450.degree. C. which is in a range between the melting point of the cobalt metal and the eutectic point of the cementing component. In a liquid phase sintered composite of the TiC--Ni--Mo system, the eutectic point of the cementing metal component is about 1270.degree. C., so that the sintering of the composite usually takes place at a temperature less than 1450.degree. C. which is the melting point of nickel metal.
As aforesaid, when these sintered bodies are produced, the sintering temperatures are below the melting points of the cementing metals in many cases. The time required for the cementing metal component to melt and change into a liquid phase while being heated and held at a sintering temperature is governed by the composition change of the cementing metal due to the solid phase diffusion of the elements constituting the hard refractory material. Thus, the time may vary depending on the manner in which the raw materials in powder form are mixed with each other, the manner in which powder bodies of the raw materials are in contact with each other and the grain size of the cementing metal powder.
On the other hand, in producing a liquid phase sintered composite body by sintering, it would be difficult to keep a compact body of a mixture of hard refractory material and a cementing metal in its original shape if the amount of the cementing metal contained therein were very large, because the cementing metal component of the compact body is melted in the process of sintering, thereby making it impossible to produce a sintered body of the desired shape. Thus, restrictions are placed on the amount of the cementing metal contained in the compact body.